Add --seed, --read_range to db_bench

Add --seed, --read_range to db_bench

Summary:
Adds the option --seed to db_bench to specify the base for the per-thread RNG.
When not set each thread uses the same value across runs of db_bench which defeats
IO stress testing.

Adds the option --read_range. When set to a value > 1 an iterator is created and
each query done for the randomread benchmark will do a range scan for that many
rows. When not set or set to 1 the existing behavior (a point lookup) is done.

Fixes a bug where a printf format string was missing.

Test Plan: run db_bench

Reviewers: dhruba

Reviewed By: dhruba

CC: leveldb
Differential Revision: https://reviews.facebook.net/D7749